1. Field of the Invention
The present invention relates to a direct fuel injection engine comprising a fuel injector which injects a fuel directly into an engine combustion chamber.
2. Related Art
Conventionally, as shown in Japanese Patent Un-examined Application Publication No.7-119507, in a direct fuel injection ignition engine in which the fuel injection is started at a later stage of a compression stroke in a low engine load condition to implement a stratified combustion, whereas the fuel injection is started at an earlier stage of an intake stroke in a high engine load and low engine speed condition to implement a uniform combustion, the fuel injection is executed several times by dividing a fuel amount to be injected so as to reduce the fuel amount of an single injection performance and facilitate the diffusion and atomization of the injected fuel to thereby accomplish a uniform combustion condition while suppressing an undesirable smoke production.
In the engine as described in the publication, in the high engine load and low engine load condition, the fuel is injected plural times by dividing the total fuel amount to facilitate the uniform combustion to thereby prevent the smoke production in the exhaust gas. There is a desire to improve a fuel consumption efficiency in the case where the fuel is injected in the compression stroke in order to accomplish a stratified combustion when the engine operation is in a low engine load and speed condition.
In this case, in a relatively high engine load condition in which a stratified combustion is still desirable, the fuel amount is increased so that the injected fuel is excessively concentrated around the ignition plug to deteriorate the mixing effect with air and thus deteriorate the combustion efficiency. In another aspect, in a relatively high engine speed condition within a stratified combustion operating zone, a time period from the fuel injection to the ignition timing is reduced. As a result, an enough time period for atomizing the fuel cannot be obtained to cause incomplete combustion to increase the amount of CO and HC in the exhaust gas.
In order to obtain an enough time to provide an improved mixing effect of fuel and air and to vaporize and atomize the injected fuel, it is proposed to advance the injection timing. However, if this is made, the fuel would be injected before the piston is stroked up to a proper level so that the injected fuel is not properly caught by a cavity formed on a top surface of the piston. As a result, the air fuel mixture gets lightened to deteriorate a combustion stability. In order to deal with the above problem, conventionally, the stratified combustion condition is limited. Thus, the fuel consumption cannot be properly improved.
In the engine as disclosed in the publication, the fuel injection is divided to be executed in the intake stroke and the compression stroke in a medium engine load operating condition to avoid an excessive fuel concentration around the ignition plug and to facilitate the vaporization and atomization of the injected fuel in the intake stroke before the ignition timing. However, where the fuel is injected in the intake stroke, the injected fuel tends to be excessively dispersed and to be lightened. As a result, the combustion efficiency is deteriorated.
In view of this, it is proposed that the fuel injection amount is increased to prevent the injected fuel from being undesirably lightened. If this is made, however, the fuel amount injected in the compression stroke is unavoidably reduced and thus the mixture around the ignition plug is lightened to deteriorate the combustion stability.